1. Field of the Invention
The present invention relates to a digital broadcast system and more particularly, to a system which allows for changing the content elements interactively in response to the viewer operation.
2. Description of the Related Art
0. Outline of the Satellite Broadcast System
First, the outline of the satellite broadcast system is to be described.
0.1. Radio Wave Transmission Status in the Satellite Broadcast
FIG. 1 is a schematic illustration of the radio wave transmission status in a satellite broadcast. The radio wave from an earth station 2 is transmitted to earth via a broadcast satellite 4. A plurality of transport streams TS1, TS2, and TS3 are transmitted from the broadcast satellite 4. The transport streams (MPEG-2) are distinguished according to the frequency, plane of polarization, and the like. The transport stream TS1 has a plurality of services (corresponding to channels of the ground wave broadcast) SV11, SV12, SV13, and SV14 provided in packets and multiplexed by time division. Likewise, the transport stream TS2 has services SV21, SV22, SV23, and SV24 multiplexed, with the transport stream TS3 having services SV31, SV32, SV33, and SV34 multiplexed. Incidentally, each of the transport streams carries video and audio data for each service as well as control data for indicating program information, control data for indicating current time, and control data necessary for packets, etc. In FIG. 1, only three transport streams are shown, however, more transport streams are transmitted in practice. Additionally, in FIG. 1, each transport stream has four services multiplexed, however, more services are multiplexed in practice.
0.2. Configuration of Transmitter
As shown in FIG. 2, each transport stream is generated and transmitted by means of a transmitter 1. Referring to the figure, only the transport stream TS1 is shown and the other transport streams TS2 and TS3 are generated in the same way.
Video/audio data ES11, ES12, ES13, and ES14 of the services SV11, SV12, SV13, and SV14 are provided to the transmitter 1. The transmitter 1 converts these video/audio data ES11, ES12, ES13, and ES14 into packets for multiplexing. The transmitter 1 also generates control data for multiplexing packets. The control data for multiplexing packets are employed for discriminating correctly video/audio data of a plurality of services in packets processed by time division. Packetized and multiplexed video/audio data (Contents) are transmitted as transport streams together with control data.
0.3. Configuration of Transport Stream
As shown in FIG. 3, the transport stream TS1 generated by the transmitter of FIG. 2 comprises multiplexed video data ES (V) 1 and audio data ES (A) 1 of the service SV11, video data ES (V) 2 and audio data ES (A) 2 of the service SV12, video data ES (V) 3 and audio data ES (A) 3 of the service SV13, and video data ES (V) 4 and audio data ES (A) 4 of the service SV14.
Furthermore, control data NIT, PAT, PMT1, PMT2, PMT3, and PMT4 for packet multiplexing are multiplexed. Video/audio data of each of the multiplexed services SV11, SV12, SV13, and SV14 can be separated by the control data. Packetization is carried out as shown with a vertical line 18a of FIG. 3. That is, packetization is performed in the order of control data NIT, PAT, EIT, TDT, ECM, video data ES (V), and audio data ES (A). After packetization is completed up to the audio data ES (A), packetization is then repeated again for the control data NIT and so forth (see a vertical line 18b).
FIG. 4 shows a basic configuration of packetized data. Control data and video/audio data are made into a packet with the configuration shown in FIG. 4. A packet ID (PID) is provided on the head of the packetized data. The packet ID is a unique symbol provided for each packet to identify individual packets. The data contents are the data (control data, video/audio data, etc.) targeted for packetization.
FIG. 5 shows the relationship between the control data PAT, PMT1, PMT2, PMT3, and PMT4, and video/audio data ES of each service, in the transport stream TS1. For example, video/audio data 500 into which the service SV12 is encrypted is packetized and provided with PID 502. Referring to the figure, though one packet is schematically shown, the video/audio data 500 is time-divided and transmitted in a number of packets.
The packet ID of the video/audio data 500 of the service SV12 is described in the control data PMT2. Accordingly, the packet ID of the video/audio data 500 of the service SV12 can be known by obtaining the contents of the control data PMT2. This control data PMT2 is also packetized and provided with PID512. In the figure, though one packet is schematically shown, the control data PMT2 is also time-divided and transmitted in a number of packets.
The packet ID of the control data PMT2 is described in the control data PAT. Accordingly, the packet ID of the control data PMT2 of the service SV12 can be known by obtaining the contents of the control data PAT. This control data PAT is also packetized and provided with PID522. Incidentally, the control data PAT has the description of packet IDs of PMT1, PMT2, PMT3, and PMT4 of services SV11, SV12, SV13, and SV14, which are multiplexed in the transport stream TS1.
A transport stream has control data and video/audio data which are packetized and associated as in the foregoing. Thus, in order to identify the services multiplexed in a transport stream, the control data PAT must be obtained first. For this reason, the packet ID of the control data PAT is fixed to a predetermined value (000 in hexadecimal notation, designated hereinafter ×0000.
Incidentally, transmission parameters (such as frequency) of each of the transport streams TS1, TS2, and TS3, and kinds of services multiplexed in each of the streams are described in the control data NIT (see FIG. 6). Accordingly, it can be known in which transport stream a particular service is multiplexed, by obtaining the contents of the control data NIT. The packet ID of the control data NIT is described in the control data PAT.
0.4. Configuration of Receiver
FIG. 7 shows the outline of a receiver. A tuner 22 selects a transport stream, and a transport decoder 26 separates the video/audio data ES relating to the desired service. Incidentally, MPU 28 sets the packet ID of the video/audio data ES of the desired service to the transport decoder 26. This allows the transport decoder 26 to output the video/audio data ES of said service. In addition, in the case where the packet ID of control data is set to the transport decoder 26, the separated control data is given to the MPU 28.
Assuming that the service SV33 of the transport stream TS3 is being received, the operation to be performed in the case where the command of switching to the service SV12 of the transport stream TS1 is given to the MPU 28 is to be explained below. First, the MPU 28 controls the transport decoder (that is, by setting the packet ID of the control data NIT) to take NIT in. The description of the NIT teaches that the service SV12 desired for reception has been multiplexed in the transport stream TS1 (see FIG. 6).
Then, the MPU 28 controls the tuner 22 to receive the transport stream TS1. Moreover, the MPU 28 controls the transport decoder 26 to obtain the PAT and PMT2, obtaining the packet IDs of the video data ES (V) 2 and the audio data ES (A) 2 of the desired service SV12 (see arrows α and β of FIG. 5).
Subsequently, the MPU 28 sets these packet IDs to the transport decoder 26 to allow the transport decoder 26 to output the video data ES (V) 2 and the audio data ES (A) 2 of the desired service SV12. As mentioned above, services are switched for reception.